After Kororaa announced its GPL problems and me writing a column about it, the people behind Kororaa have now posted an in-depth follow-up: "I have been receiving lots of information which I have been sorting through, thank you to everyone who has emailed me (although I would have also thanked you personally via email). I contacted both ATI and nVidia for some clarification on particular issues, however neither have answered my questions. Nevertheless, this is what I have found so far."

That's actually an interesting question, and potentially the crux of the matter. The Linux header files are, arguably, not purely descriptive. They contain a large number of inline functions and functional macros. Some of these are sufficiently complicated that they probably merit copyright protection, and it's pretty much impossible to write a Linux driver without using some of them.

If so, it seems difficult to make the argument that any Linux driver isn't a derived work of the Linux kernel - using an inline function means that the (presumably copyrightable and GPLed) code is copied into your binary. But, again, there's no way to know for sure without a lawsuit.